A US woman who shot and killed her husband and two adult children before taking her own life is thought to have committed the shocking crime as a result of being ostracised from the religion she was raised in.

A federal judge sentenced a former Arkansas judge Wednesday to five years in prison — a stiffer punishment than prosecutors recommended — after he admitted giving young male defendants lighter sentences in return for personal benefits that included sexual favours.

Every time I tell a mate I’m doing a story on cryptocurrency, they invariably ask me the same two questions: should they invest their own hard-earned money, and which cryptocurrency will get them a Lamborghini/yacht/island quickest?

In a 60 Minutes online exclusive, reporter Liz Hayes quizzed Australian Prime Minister Malcolm Turnbull on his relationship with the unpredictable Twitter aficionado and US president Donald Trump ahead of their meeting at the White House Friday.

Climate change mitigation was a focus during Prime Minister Tony Abbott's recent visit to the White House, where US President Barack Obama said he accepted the government's mandate to repeal the carbon tax.

But Mr Obama urged Australia and other nations to adopt "ambitious domestic climate policies as the basis of a strong international response".

"The big point here is that the carbon tax hasn't been doing its job," Mr Hunt told Seven Network on Sunday.

"Why did the Australian people vote to get rid of it? Because you had a $7.5 billion tax on electricity and gas ... [and] emissions went down by 0.1 percent in the first full year of the carbon tax."

New modelling by energy advisory firm RepuTex suggests Australia can expect to fall well short of its target of five per cent emission reductions by 2020.

Its analysis predicts that by 2020 the emissions reduction fund alone will purchase between 30 and 120 million Australian carbon credit units, leaving a carbon shortfall of more than 300 million tonnes.

The assurance comes as Fairfax Media reports Mr Hunt was forced to back down from a promise to the Clean Energy Council last November that the coalition was still committed to its $500 million "1 million solar roofs" program, a policy leftover from the 2010 election.

Mr Hunt reportedly described the flagship solar rebate program as a "shining beacon" of the government's direct action climate policy - though the policy had not been reaffirmed by Prime Minister Tony Abbott.

Asked to respond to the report, Mr Hunt said: "We've added $1 billion during the course of the budget process to the emissions reduction fund ... we've had to make some difficult choices."

The solar "debacle" is the latest in a string of broken government promises on renewable energy policy, opposition climate change spokesman Mark Butler said.

"When Greg Hunt talked about this the Australian people [and] the solar industry were of the view that the renewable energy target was a bipartisan position," he told Sky News.